The field of the present invention is bearing support systems for high speed rotating machinery.
Conventional journal bearings that are lubricated with viscous lubricants and used to support a rotating shaft have the characteristic of allowing the shaft to displace itself under bearing load in proportion to the force of the load. Thus, the bearing has a so-called spring rate. The rotating assembly also has a mass, so the combination of this mass and the spring rate of the bearing comprises the elements of an oscillating system the characteristics of which may be derived mathematically. The system characteristic of concern here is a so-called critical speed of the shaft and bearing system. This critical speed is the rotational frequency at which the shaft will resonate in the bearing. Frequently, it is very difficult to get sufficiently rigid bearings to place this resonance frequency above the shaft running speed, and thus it often occurs within the operating range. The result can be severe vibration and possible bearing damage.
One solution to this problem is suggested by Sternlicht, U.S. Pat. No. 3,124,395. Sternlicht discloses a method and apparatus for bypassing critical speeds of rotors by varying either the lubricant film thickness, lubricant viscosity or the lubricant temperature in hydro-dynamic bearings at predetermined shaft speeds. Sternlicht's method, however, is somewhat complicated, would appear to suffer from rather slow response time, and being responsive only to predetermined speeds, would not be readily adaptable to varying shaft loading conditions that would alter the critical speeds. Thus, it is desirable to have a journal bearing support system with a controllable spring rate which is highly responsive to the onset of a critical speed condition and which is operable under variable loading conditions.